Refactor hiding-methods, remove jQuery show/hide, remove .hide class, remove inline style=display:none (#22950)

Close #22847

This PR:

* introduce Gitea's own `showElem` and related functions
* remove jQuery show/hide
* remove .hide class
* remove inline style=display:none 

From now on:

do not use:
* "[hidden]" attribute: it's too weak, can not be applied to an element
with "display: flex"
* ".hidden" class: it has been polluted by Fomantic UI in many cases
* inline style="display: none": it's difficult to tweak
* jQuery's show/hide/toggle: it can not show/hide elements with
"display: xxx !important"

only use:
* this ".gt-hidden" class
* showElem/hideElem/toggleElem functions in "utils/dom.js"

cc: @silverwind , this is the all-in-one PR

templates/repo/editor/edit.tmpl

@@ -36,7 +36,7 @@
 					{{end}}
 				</div>
 				<div class="ui bottom attached active tab segment" data-tab="write">
-					<textarea id="edit_area" name="content" class="hide" data-id="repo-{{.Repository.Name}}-{{.TreePath}}"
+					<textarea id="edit_area" name="content" class="gt-hidden" data-id="repo-{{.Repository.Name}}-{{.TreePath}}"
 						data-url="{{.Repository.Link}}/markdown"
 						data-context="{{.RepoLink}}"
 						data-markdown-file-exts="{{.MarkdownFileExts}}"

templates/repo/editor/patch.tmpl

@@ -25,7 +25,7 @@
 					<a class="active item" data-tab="write">{{svg "octicon-code" 16 "gt-mr-2"}}{{.locale.Tr "repo.editor.new_patch"}}</a>
 				</div>
 				<div class="ui bottom attached active tab segment" data-tab="write">
-					<textarea id="edit_area" name="content" class="hide" data-id="repo-{{.Repository.Name}}-patch"
+					<textarea id="edit_area" name="content" class="gt-hidden" data-id="repo-{{.Repository.Name}}-patch"
 						data-context="{{.RepoLink}}"
 						data-line-wrap-extensions="{{.LineWrapExtensions}}">
 {{.FileContent}}</textarea>